1. Technical field
The present invention relates to a printed wiring board and its manufacturing method, and particularly relates to joining of the printed wiring board and a mother board.
2. Background art
For example, as shown in FIG. 27, there is conventionally a printed wiring board having a circuit substrate 96 having a mounting portion 97 for mounting an electronic part 970 thereto, a conductor circuit 95 arranged on a surface of the circuit substrate 96 and also arranged within this circuit substrate 96, and a through hole 93 extending through the circuit substrate 96.
The electronic part 970 is electrically connected to the conductor circuit 95 by a bonding wire 971. As shown in FIGS. 27 and 28, a pad 92 for mounting a ball thereto is arranged at an end tip of the conductor circuit 95 on a rear face side of the circuit substrate 96. A soldering ball 91 is joined to a surface of this pad 92.
The above conventional printed wiring board 9 is joined to an opposite party pad 981 arranged on the surface of a mother board 98 by heating and melting the above soldering ball 91.
The printed wiring board fulfills a function for transmitting electric information of the electric part to a partner member such as the mother board, etc. by the above structure.
However, the above conventional printed wiring board has the following problems. Namely, as shown in FIGS. 27 and 28, the printed wiring board 9 is fixedly joined onto the mother board 98 by the soldering ball 91. Therefore, the pad 92 for mounting the soldering ball must be arranged in the printed wiring board 9.
Further, it is necessary to arrange the conductor circuit 95 between the pad 92 for mounting the ball and the through hole 93 and electrically connect the pad 92 and the through hole 92. Therefore, as shown in FIG. 28, the rear face side of the circuit substrate 96 is occupied by the pad 92 for mounting the ball and the conductor circuit 95 connected to the through hole 93 as well as the through hole 93. Therefore, it is difficult to secure a sufficient space for arranging another conductor circuit on the rear face side of the circuit substrate 96 so that high density wiring is prevented.
When the soldering ball 91 is joined to the opposite party pad 981, it is difficult to control a melting state of the soldering ball 91. Namely, the soldering ball 91 is melted and joined by heating this soldering ball 91 to the opposite party pad 981 on the mother board 98. At this time, the melting state of the soldering ball 91 is different in accordance with elements such as heating temperature, applied pressure, soldering composition, etc. Therefore, as shown in FIG. 27, there is a case in which a melting degree of the soldering ball 91 on one side (e.g., a right-hand side of FIG. 27) of the printed wiring board 9 is increased and the melting degree of the soldering ball 91 on the other side (e.g., a left-hand side of FIG. 27) is decreased. In this case, the printed wiring board 9 is slantingly joined to the mother board 98. Accordingly, it is difficult to join the conventional printed wiring board 9 to the mother board 98 in parallel with this mother board 98.
In addition, as shown in FIG. 29, the pad 92 for mounting the ball and joining the soldering ball 91 is generally covered with a gold plating film 921. A gold component of the gold plating film 921 permeates the interior of the soldering ball 91 in heating and melting the soldering ball 91, and forms an intermediate layer 90 between the metallic plating film 921 and the soldering ball 91.
This intermediate layer 90 has a property deteriorated by heat. Therefore, when the intermediate layer 90 is formed, joining strength of the pad 92 for mounting the ball and the soldering ball 91 is reduced until 1.0 to 1.4 kg/cm2.
Further, the intermediate layer 90 is increased in thickness as the gold plating film 921 covering the pad 92 for mounting ball is increased in thickness. Therefore, when the thickness of the gold plating film 921 is increased, the joining strength of the pad 92 for mounting the ball and the soldering ball 91 is further reduced. Accordingly, there is a case in which the joining strength is reduced to a joining strength equal to or smaller than 1.0 kg/cm2.
With consideration of such conventional problems, the present invention provides a printed wiring board able to perform high density wiring on a substrate surface and able to be joined to a partner member in parallel with this partner member and having an excellent joining strength, and also provides a manufacturing method of the printed wiring board.